a) Field of the Invention
The invention is directed to a projection device with a matrix located in an image plane for generating a video image, with optics for projection of this video image on a screen and with light source for illuminating the matrix.
b) Desecration of the Related Art
A technique of the type mentioned above is known from video projection devices, already commercially available at the present time, in which an LCD matrix is installed between a light source and optics like a transparency in a slide projector and is imaged by the optics on a screen.
The LCD matrix is controlled, for example, for displaying video images. In this way, a video picture can be imaged on a screen as a large image by means of the technique known from slide projection. This large-image technique is considered to be trend-setting because electronic picture tubes can not be used for very large images.
Aside from the above-mentioned transmitted light projection known for transparencies, incident light projection with an LCD such as is known from episcopes would also be suitable for the above purpose, wherein the LCD lies in the image plane of the episcope construction in this case.
When use of the incident light projection method is desirable, a mirror array or matrix can also be used instead of the LCD matrix. A matrix of this kind can be obtained as a circuit from Texas. Instruments, for example. In this case, a plurality of tilting mirrors, one for each picture point, arranged in a matrix are digitally controlled. In one digital state, a tilting mirror reflects the full light intensity; in the other state, the mirror receives and reflects the light at an angle at which it can no longer be thrown onto the screen. This means that the corresponding picture point is dark on the screen except for small proportions of scattered light.
The different degrees of brightness of the light for displaying the gray value or color value of a picture point are adjusted in that the mirrors are acted upon by a suitable pulse train so that only an intermediate value between full light intensity and dark is perceived by the eye of an observer for every picture point in time average.
The known episcope method and slide projection methods are disadvantageous in that a very high light output is required of the light source, whose heat output could, for example, destroy the LCD. For this reason, elaborate cooling is also required in projectors of this type which makes these devices heavy, inconvenient and expensive.
In addition, conventional episcope technique is not suitable for the tilting mirror technique because with the latter technique the best image is achieved when the light reaches the matrix only at a defined angle, i.e., precisely the adjustment angle of the tilting mirrors for the darkness value such that only a small surface of practically zero is opposed to the light source from the mirror for reflection. However, in the case of a concave-mirror structure such as is known from episcopes for illumination of an image, the defined angle cannot be adjusted. Therefore, as regards illumination which is advisably carried out at a defined angle in the case of tilting mirrors, a substantially greater light loss must be taken into account compared with the known episcope technique.
It is the primary object of the invention to reduce light loss in such projectors as far as possible and accordingly to provide a compact video device of the type mentioned above in which the problem of cooling is minimized in spite of a compact construction.
This object is met by providing an arrangement by which the video image is uniformly illuminated and, in particular, in that a device for mixing the light of the light source by means of multiple reflections is provided between the light source and the matrix, and the mixed light is directed to the matrix proceeding from the device.
The device, according to the invention, for mixing the light of the light source is used for generating the most uniform possible light density or luminance on the matrix. In conventional lamps there is always nonuniform luminance because of the filament or the volume always required for emission of light, so that a certain nonuniformity must always be taken into account in illumination. For this reason, the generated light spot on the transparency, on the image to be projected, or on the LCD matrix should always be expanded farther than the dimensions of the latter so as to enable a light field which is as homogeneous as possible. However, the light loss resulting from this is reduced according to the invention in that the light is repeatedly reflected back and forth in the device for mixing before it exits this device again, so that the origination of every light bundle in the emission volume of the light source is lost. The light field generated in this way then has a very uniform luminance after exiting the device for mixing. This device for mixing can be, e.g., a light-conducting fiber or a mirror system for reflecting back and forth.
The resulting advantage for mixing, particularly with respect to required output, can easily be illustrated by a numerical example. While conventional technique only assumes that an area twice as large as that necessary for the area of the image must be illuminated, the edge length can be reduced by up to one half in the uniform illumination according to the invention. This means that only a fourth of the output is required as a result of the device for mixing.
The extreme benefit gained by the device for mixing according to the invention derives above all in the improved adaptation of the size of the light spot to the area of the image. The drastic reduction in output achieved in this way follows from the quadratic dependence of the size of the light spot on the edge length to be illuminated.
However, it has also turned out unexpectedly that the video images generated by means of the device for mixing are of a substantially higher quality than is the case without the device. This results primarily from the fact that the more uniform illumination of the matrix carried out by means of the invention also permits a better reproduction of the image contents.
In a preferred further development of the invention, the matrix is formed of digitally controllable tilting mirrors. In comparison with the LCD matrixes mentioned above, the benefits of the device for mixing come into play in a particularly advantageous manner precisely in these matrixes by which a defined angle can also be adjusted because the light impinges essentially only from the direction of the device for mixing.
As was already stated, it is advisable in mirror matrixes that a defined angle can be maintained in a particularly simple manner by means of a corresponding geometric construction of the projection device and a suitable geometric arrangement of the device for mixing with respect to the matrix.
The light loss can then be further reduced, according to an advantageous further development of the invention, when coupling-in optics are provided between the light source and the device for mixing. The coupling-in optics ensure that the greatest possible amount of light reaches the device for mixing from the light source. Therefore, it also prevents an avoidable light loss.
In the same way, another further development of the invention advantageously results in a reduction in the required amount of light in that coupling-out optics are provided between the device for mixing and the matrix, wherein the matrix can be illuminated by the coupling-out optics. In particular, the angle for the illumination of the matrix with the digitally controllable tilting mirrors can be adjusted with the coupling-out optics by means of a corresponding geometric configuration in such a way that the highest possible contrast for high-quality images can be achieved.
It is further provided according to a preferred further development of the invention that a field lens is arranged in front of the matrix. By means of the field lens, it is possible, for example, to achieve a magnification effect by means of which the light exiting from the device for mixing can be adjusted particularly well to the image field to be illuminated, since the accuracy of adjustment is increased as a result of the magnification effect of the field lens. This makes it easier to bring the light spot into coincidence with the image field. Due to the resulting simplification, it is advantageous, above all for series production, that the light field for illuminating the matrix can be provided with especially small dimensions Accordingly, this feature also has a positive effect with respect to economizing on output.
Scattering mirror surfaces followed by focusing optics could be used as the device for mixing. Light-conducting fibers could also be used for this purpose, but would have to be rigidly fastened so that the image field can shift if their position changes. However, it has also proven advantageous in a further development of the invention when the device for mixing has a body in the form of a geometric prism, wherein the light from the light source is introduced into the base surface of the prism and the mixed light for illuminating the matrix is taken from the upper surface of the prism. In this way, an effective mixing is generated during the transport of light over the length of the geometric prism, while the light is reflected back and forth at its side surfaces. In particular, this further development of the invention also has the advantage that bodies of the above-mentioned type can be manufactured in a simple manner so that arrangements according to the invention intended for the consumer market are especially economical.
In an advantageous further development of the invention, the geometric prism has a rectangular base surface whose aspect ratio is determined by the aspect ratio of the video image formed by the matrix.
In this respect, a particularly uniform illumination is achieved in all directions due to the fact that the light emerging from the upper surface allows the two aspect ratios of the upper surface of the prism shape to be adapted to those of the video image. However, it must be taken into account with respect to the adaptation of the two aspect ratios that the aspect ratio could be changed by mirrors, diaphragms, lenses, etc. in the light path behind the device for mixing the light emerging therefrom. In this connection, the aspect ratios are determined in such a way that the light exiting from the body uniformly illuminates the matrix in both surface dimensions A body of this kind could be constructed, for example, as a tube with inner reflective coating. In this way, practically all of the light entering the device for mixing at its input can be transmitted to the output. However, it has proven substantially more advantageous when the outer surface of the body used for mixing is not given a reflective coating and mixing is carried out by means of multiple reflections based on total reflection at this outer surface. In this way, the reflective coating can be dispensed with, which is extremely advantageous in the interests of an inexpensive video device especially for the consumer market.
In a preferred further development of the invention, the ratio of the length of the prism axis to the lateral extension is greater than
5/{square root over (n2xe2x88x921)},
where n represents the calculation or optical design index of the material. This equation makes use of the insight that a mixing which is sufficient for video projection is carried out when the light is reflected back and forth at least five times. Within the indicated limit, the smallest length for the prism-shaped body can be determined in order to achieve an effective device for mixing while maintaining small dimensions at the same time. That is, the dimensions should not be too large so that the projection device remains compact. It is therefore advisable with respect to the dimensioning of the prism length of the mixing device not to exceed 100 to 200 reflections. Further, the polygonal body is advantageously made of glass.
It is precisely in the case of total reflection, however, that light losses can occur as a result of incorrect holding of the prism, wherein these light losses can lead to partial disruption of the total-reflection behavior due to bending or contact between the holder and the polygonal body. In order that light loss of this kind can also be minimized as far as possible, it is provided in accordance with a further development of the invention that a holder for the body is made from sheet metal with a thickness of less than 1 mm, in particular, less than 0 5 mm, wherein the holder contacts the body exclusively by an edge having this dimension. Because of the small contact surface of the holder with respect to the polygonal body which is made possible in this way, light losses are negligible at the given dimensions.
Further, an appropriate holder is characterized according to an advantageous further development of the invention in that a holder for the body is made from springing sheet metal whose spring tension acts vertical to the prism axis and draws the body vertical to the prism axis against an edge of the housing as an abutment and in that a groove is provided in an outer surface of the body, wherein a spring which is attached to the housing engages in this groove.
It is also possible by means of matrixes of the kind mentioned above to display a color image display. One possibility consists in providing a color wheel with different color filters between the light source and the matrix, which color wheel sequentially filters different colors of the light due to its rotation, wherein the information is adjusted on the matrix synchronously with the color of the light impinging on the matrix at that instant. In a preferred further development of the invention, this color wheel is arranged between the light source and the device for mixing. This position is particularly suitable because scattering in or on the color wheel due, for example, to imperfections in the material or because of surface dust is compensated by the subsequent mixing, so that a high-quality video image is possible even when such defects occur.
According to a preferred further development of the invention, it is provided that an adjusting element is arranged behind the device for mixing so that the light emerging from the device for mixing is oriented on the matrix.
As was already made clear, for prevention of light losses especially with respect to the small surfaces intended to be illuminated with tilting mirror matrixes, it is particularly advantageous when the light emitted from the device for mixing can be adjusted on the matrix. An individual adjusting element whose construction will be made clearer from the embodiment example described in the following is provided for this purpose.
The following remarks pertain primarily to tilting mirror matrixes. Because of the small divergence required for complete absorption when a video image is darkened and the simultaneous desire for high light intensity when brightening so that a large screen can be illuminated, problems arise in a compact construction because, on the one hand, a long light path must be provided and, on the other hand, the output loss should not lead to a heat build-up in the component elements for reasons of economy. Particularly as regards heat management, it must be taken into account that the tilting mirror matrix is a micromechanical semiconductor component which operates in a reproducible manner only within a defined temperature range.
In a preferred further development of the invention, the light source is arranged near the tilting mirror matrix and at an angle such that the light bundle is directed away from this tilting mirror axis, and a folded optical light path is provided for transmitting a light bundle emitted by the light source to the tilting mirror matrix.
By means of the folded optical light path, the appropriate length is adjusted so that the light impinging on the tilting mirror matrix has a suitably small divergence. However, it serves another purpose according to the invention in that the light of the light source which, in accordance with the invention, is not directed onto the tilting mirror matrix directly is deflected for displaying images.
There are two distinct advantages in arranging the light source in the vicinity of the tilting mirror matrix, wherein the light bundle is directed away from the tilting mirror matrix. First, cooling, e.g., by means of a ventilator, can be applied in such a way that the light volume, especially a filament, of the light source, and the tilting mirror matrix can both be cooled by a single element. Second, the cooling leads to very stable reproducible video images since all of the heat loss originates in the vicinity of the tilting mirror matrix, which means that switching the tilting mirrors to other states, for example, for displaying a darker video image, only leads to a slight change in the heating of the tilting mirrors due to the resulting increased absorption, so that the tilting mirror matrix is kept essentially temperature-stabilized through cooling and uniform heating by means of the light source. This means that, in contrast to other arrangements, the reproducibility of the images is substantially independent from the image content. Therefore, a particularly compact video device can be provided.
In order to keep the required amount of light as small as possible so that heat-related problems are further reduced, it is provided in an advantageous further development of the invention that the folded optical light path is brought about by two deflecting mirrors, wherein a light guide extends therebetween. Costs are lowered by limiting essentially to two deflecting mirrors. The provided light guide provides in particular for a high homogeneity of light, so that the tilting mirror matrix can be uniformly illuminated. This leads to a lower light output for the light source because, in principle, only the surface area of the tilting mirror matrix must be illuminated. Further, the light from the light guide is homogenized because of the back-and-forth reflection; that is, a more uniform light spot is provided for illuminating the tilting mirror matrix, which enables an especially high image quality.
In a preferred further development of the invention, the light guide is a rectangular rod made of material which is transparent for the light of the light source and has a cross section adapted to the tilting mirror matrix. In a rectangular rod of this kind the light is conducted by total reflection as is also known from other light guides. While a rod of this kind could also be constructed with mirror coatings, a simple rod can be manufactured in a particularly economical manner, for example, from glass, resulting in a lower price which is particularly advantageous for the consumer market.
The cross section adapted to the tilting mirror was selected for this further development so that virtually all of the light emerging from the rod can be used to illuminate the matrix by imaging the output face of the rod on the tilting mirror matrix As a result of this step, a light source can be used which has a lower output than would be needed if the edges of the tilting mirror matrix had to be illuminated by large areas. This step also drastically reduces problems related to heat.
For the same purpose, according to an advantageous further development of the invention, coupling-in optics are provided between the light source and the rectangular rod for focusing the light bundle on an input face of the light guide. Due to this step, as much of the light from the light source as possible is coupled into the light guide, so that the output requirement of the light source can be lowered due to the resulting reduced light losses, which in turn reduces heat problems.
According to another preferable further development of the invention, a color wheel is provided between the coupling-in optics and the input face of the light guide. A color wheel is known with respect to the described generation of video images with tilting mirror matrixes for displaying color pictures. With this color wheel, different color filters are switched in front of the light source in sequence, so that different colors proceed from the light source. The tilting mirror matrix is controlled synchronously with the respective light color with the different color separations, e.g., red, green, and blue (R, G, B). The impression of a color image originates through superposition of the color separations as a result of the inertia of the human eye.
With respect to this further development of the invention, the arrangement of the color wheel is particularly advantageous. Because of its position between the coupling-in optics and input face of the light guide, it is far away from the parts of the arrangement that are subjected to heat, as will be seen in particular from an embodiment example. In the embodiment example, an air flow for cooling the light source and tilting mirror matrix can also be directed to the color wheel simultaneously. Further, a particular advantage results in this arrangement in that variations in light intensity due to irregularities in the color wheel and dust on its surface are rendered homogeneous by the light guide, which results in a particularly high image quality.
The light bundle can be aligned particularly well for reduction in the output of the light source according to a preferred further development of the invention when a cylindrical adjusting element is provided in the folded optical light path, wherein a deflecting mirror of the folded optical light path is fastened in the adjusting element, wherein the light of the light source can be oriented on the tilting mirror matrix by means of this deflecting mirror. For a particularly good orientation to the tilting mirror matrix, it is further advantageous when all of the light is focused on the tilting mirror matrix. For this purpose, it is provided according to a preferred further development of the invention that an optical system for focusing the light bundle on the tilting mirror matrix is arranged in the adjusting element.
As was already made clear above with respect to the rectangular rod, it is especially advantageous when a rectangular light surface is imaged on the tilting mirror matrix. Possible focusing for this purpose can be carried out, for example, with the optical system which is arranged inside the adjusting element. By changing the location of the adjusting element during adjustment, suitable focusing conditions can always be adjusted.
In this regard, it is also provided in a preferred further development of the invention that a rectangular, light-emitting surface supplied by the light source is formed in front of the adjusting element and the adjusting element has at least a two-lens system by means of which the light-emitting surface can be imaged on the tilting mirror matrix.
The rectangular surface can be constructed by means of the rectangular rod mentioned above. It is also possible for the surface supplied by the light source to be formed in other ways, for example, by means of providing a rectangular diaphragm in the beam path.
According to a preferred further development of the invention, it has turned out to be particularly advantageous for compactness and for good imaging properties at the same time that the linear magnification or imaging scale of the at least two-lens system is between 1 and 5. In the embodiment example indicated hereinafter, a reduction factor of 2 was selected in particular.
A favorable light output combined with a low heat output results from an improved adjusting possibility according to an advantageous further development of the invention in that a field lens is arranged in front of the tilting mirror matrix. The adjustment is facilitated in particular because the field lens also maintains the desired angle on the tilting mirror matrix, that is, essentially the angle for optimum dark/bright ratios. Further, adjustment is facilitated because the field lens enables a large movement of the adjusting element into a small movement of the light bundle on the tilting mirror matrix. This improved adjustment possibility is especially beneficial for a commercial video device because the light path can be adjusted more easily even when the device is constructed in a very compact manner and the light spot generated on the tilting mirror matrix by the light source can be guided in an economical manner virtually completely with only slight, acceptable overlapping of the edge area of the tilting mirror matrix.
According to a further development of the invention, it is provided in particular that the light bundle directed on the tilting mirror matrix along the optical light path is directed through the field lens to the tilting mirror matrix, while the light reflected from the tilting mirror matrix into the objective enters the objective without being influenced by the field lens. This benefits increased compactness because the field lens no longer interferes with the light leaving the tilting mirror matrix. One of the ways that this can be achieved is through a special construction of the field lens in which a hole is drilled in the lens for the light reflected by the matrix.
For a particularly economical construction of the video device, especially also to promote compactness, it is provided according to an advantageous further development of the invention that the field lens has a semicircular cross section vertical to the optical axis, especially a half-circular cross section which is arranged between the incident light bundle and tilting mirror, wherein an opening defined by the divergence from the circular shape lies between the tilting mirror matrix and the objective.